US Secretary of State John Kerry will travel to Kiev this week in a show of support for the embattled leadership, as Washington and its allies slammed Moscow for violating Ukraine's sovereignty. The top US diplomat led warnings from around the globe that Moscow risked being stripped of its coveted seat as one of the Group of Eight nations, and could face a damaging economic fallout for sending troops into Ukraine's Crimean Peninsula. Kerry denounced what he called a "brazen act of aggression", as he pointedly added a stop in Kiev to a planned trip to Rome and Paris this week. He will meet Tuesday with the new Ukrainian leadership and the parliament to "reaffirm the United States' strong support for Ukrainian sovereignty, independence, territorial integrity," State Department spokeswoman Jen Psaki said in a statement. The surprise trip reveals growing alarm at fast-moving events in Ukraine, with US senior officials saying Russian forces now had "complete operational control" of southern Crimea. Washington has renewed calls for Russian President Vladimir Putin to pull back his troops, and the leaders of the world's top industrialized powers turned on their fellow G8 member, condemning Russia's "clear" violation of Ukraine's sovereignty. Symbolically billing themselves as the "G7," the leaders said in a statement that Russia's actions were incompatible with the group's principles. And in unison, they said they would not take part in preparatory talks due this week for June's G8 summit in the Russian city of Sochi. After a 90-minute phone call with Putin on Saturday, President Barack Obama spoke Sunday with German Chancellor Angela Merkel, Britain's Prime Minister David Cameron and Polish President Bronislaw Komorowski. The leaders said Moscow's actions were a "breach of international law and a threat to international peace and security," a White House statement said. They also pledged to work together on a financial aid package for cash-strapped Ukraine. In the call with Cameron, "they agreed there must be 'significant costs' to Russia if it did not change course on Ukraine," Downing Street said. Washington and its G8 allies were prepared to slap sanctions on Moscow, Kerry said, warning billions of dollars in trade and investment could be at stake. There could also be possible visa bans and American businesses may move to pull out of Russia. "If Russia wants to be a G8 country, it needs to behave like a G8 country," Kerry told CBS television's "Face the Nation." Putin "is not going to have a Sochi G8, he may not even remain in the G8 if this continues. He may find himself with asset freezes on Russian business... there may be a further tumble of the ruble," Kerry warned on NBC's "Meet the Press." - World seeks 'peaceful resolution' - Kerry's blunt comments came as Kiev's interim leaders warned their country was on the brink of disaster after the Russian parliament voted to allow Putin to send in troops. Canada firmly denounced Russia's military incursion into Ukraine as "Soviet-style aggression," which Canadian Foreign Minister John Baird said was "completely unacceptable." Some economic talks have already been cancelled, as well as a visit by a Russian delegation to Washington to discuss energy. And a major military event due to be held soon would also likely be shelved. NATO called for the dispatch of international observers to Ukraine, while also seeking talks with Moscow. US officials are working to try to get international monitors and observers into flashpoint areas. But Kerry steered clear of warning of any US military action. "The last thing anybody wants is a military option in this kind of situation. We want a peaceful resolution through the normal processes of international relations," he said. Kerry said US economic sanctions could be imposed on Moscow, stressing "all options are on the table." And he warned on CBS that "the G8 plus some others... are prepared to go to the hilt in order to isolate Russia with respect to this invasion." Russia's economic growth slowed to 1.3 percent in 2013, down from 3.4 percent the year before, and the ruble has lost more than eight percent against the euro since the start of the year. US Treasury Secretary Jack Lew said an International Monetary Fund program should be the "centerpiece" of an international aid package for Ukraine. The United States was willing "to supplement IMF support in order to... cushion the impact of needed reforms on vulnerable Ukrainians," he added.